improve rtrace and move it to lib

lib/rtrace/README.md

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+# ![AS](https://avatars1.githubusercontent.com/u/28916798?s=48) rtrace
+
+A tiny utility that records allocations, retains, releases and frees performed by the runtime and emits an error if something is off. Also checks for leaks.
+
+Instructions
+------------
+
+Compile your module that uses the full or half runtime with `-use ASC_RTRACE=1` and include an instance of this module as the import named `rtrace`.
+
+```js
+var rtr = rtrace(e => {
+  // handle error
+});
+
+WebAssembly.instantiate(..., { rtrace: rtr, ... });
+...
+
+if (rtr.active) {
+  let leakCount = rtr.leakCount;
+  if (leakCount) {
+    // handle error
+  }
+}
+```
+
+Note that references retained in globals which are not cleared before execution concludes appear as leaks, including their inner members. A TypedArray would leak itself and its backing ArrayBuffer in this case for example. This is perfectly normal and clearing all globals avoids this.

lib/rtrace/index.js

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+function rtrace(onerror) {
+  var blocks = new Map();
+  var rtrace = {
+
+    allocCount: 0,
+    freeCount: 0,
+    incrementCount: 0,
+    decrementCount: 0,
+
+    onalloc: function(block) {
+      ++rtrace.allocCount;
+      if (blocks.has(block)) {
+        onerror(Error("duplicate alloc: " + block));
+      } else {
+        blocks.set(block, 0);
+      }
+    },
+
+    onfree: function(block) {
+      ++rtrace.freeCount;
+      if (!blocks.has(block)) {
+        onerror(Error("invalid free: " + block));
+      } else {
+        blocks.delete(block);
+      }
+    },
+
+    onincrement: function(block) {
+      ++rtrace.incrementCount;
+      if (!blocks.has(block)) {
+        onerror(Error("invalid increment: " + block));
+      } else {
+        let rc = blocks.get(block) + 1;
+        blocks.set(block, rc);
+      }
+    },
+
+    ondecrement: function(block) {
+      ++rtrace.decrementCount;
+      if (!blocks.has(block)) {
+        onerror(Error("invalid decrement: " + block));
+      } else {
+        let rc = blocks.get(block) - 1;
+        if (rc < 0) {
+          onerror(Error("invalid decrement: " + block));
+        } else {
+          blocks.set(block, rc);
+        }
+      }
+    },
+
+    get active() {
+      return Boolean(rtrace.allocCount + rtrace.freeCount + rtrace.incrementCount + rtrace.decrementCount);
+    },
+
+    get leakCount() {
+      return blocks.size;
+    }
+  };
+  return rtrace;
+}
+module.exports = rtrace;

lib/rtrace/package.json

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+{
+  "name": "@assemblyscript/rtrace",
+  "version": "0.1.0",
+  "license": "Apache-2.0",
+  "main": "index.js"
+}